1. Field of the Invention
This invention relates to a process for treating nickel-copper mattes for the recovery of copper and nickel values. More particularly, this invention pertains to a process for treating arsenic-containing nickel-copper matte to produce an essentially copper-free solution of nickel sulphate, from which metallic nickel can be recovered by electrowinning, and a copper sulphide concentrate containing low levels of both nickel and arsenic, from which copper can be recovered by smelting and refining.
2. Background of the Invention
Copper sulphide concentrates should contain less than about 1% Ni and less than about 1% As to be suitable for treatment in a conventional copper smelter to produce anode copper for electrorefining. Nickel-copper matte, from which low a nickel- and arsenic-content copper sulphide concentrate is to be produced, typically contains 65 to 70% Ni, 20 to 25% Cu, 2% As and 6% S. Currently this material is treated in two stages of atmospheric acid leaching at 80.degree. C. to produce a copper free nickel sulphate solution, and a copper rich residue. The nickel sulphate solution forms the feed to conventional cobalt removal and nickel electrowinning circuits, where the cobalt and nickel are recovered as cobaltic oxide and cathode nickel, respectively. The copper-rich residue typically contains 45 to 55% Cu, 10 to 15% Ni, 2 to 4% As, and 17 to 20% S. Since the nickel and arsenic contents are much too high for treatment of the copper sulphide directly in a copper smelter, the residue is recycled to the nickel smelter which produces the nickel copper matte.
A principal object of this invention is to provide a process for producing a copper sulphide residue containing low levels of nickel and from an arsenic containing nickel-copper matte. More particularly, an object of this invention is to treat a copper rich residue to reduce the nickel and arsenic contents to less than 1%, thus avoiding the need to recycle the residue to the nickel smelter, which is costly, and which increases metal losses to slag. Direct treatment of the copper sulphide in a copper smelter potentially offers increased metal recoveries, reduced costs, and also permits the separate recovery of precious and platimum group metals in a concentrated form.
Prior art leach processes are disclosed in U.S. Pat. No. 4,323,541 granted Apr. 6, 1982 to Outokumpu Oy and U.S. Pat. No. 3,616,331 granted Oct. 26, 1971 to International Nickel Company Inc.
U.S. Pat. No. 4,323,541 discloses leaching a finely-ground nickel-copper matte using an acid solution at a minimum temperature of approximately 80.degree. C. and under oxidizing conditions in order to produce a nickel sulphate solution and a nickel-copper sulphide residue, which is separated from the solution. Thereafter the nickel-copper sulphide residue is leached using acid in an autoclave at a minimum temperature of 110.degree. C. and under oxidizing conditions in order to produce an additional quantity of nickel sulphate solution and a copper sulphide precipitate.
U.S. Pat. No. 3,616,331 shows a two-stage process for the treatment of a nickel-containing copper sulphide material which contains copper and sulphur in ratios of at least 3.5:1, preferably between 4:1 and 10:1, and controlled amounts of iron. This material is leached with an acidic aqueous solution containing copper sulphate in amounts at least stoichiometrically equivalent to the nickel, iron and cobalt contained in the starting material, and sufficient sulphuric acid to maintain a pH below 5- Advantageously, the copper sulphate solution contains a stoichiometric excess of copper sulphate of up to about 20% in order to ensure maximum dissolution of the nickel, iron and cobalt as sulphates. The leaching operation is conducted at a temperature between about 100.degree. C. and 250.degree. C. under the pressure generated at these temperatures. The residue obtained from this leaching operation contains substantially all the copper in the starting material as copper sulphide and cement copper and is substantially free of iron. After filtration, the filtrate is treated to recover nickel, and the residue is treated to recover copper and sulphur in a second pressure leaching step.
Each of the processes described in these prior art patents was tested on a copper rich residue produced in a conventional two-stage atmosphere acid leaching process. The process of U. S. Pat. No. 4,323,541 was tested in a batch leach at 150.degree. C. under an air pressure of 150 kPa, using a solution containing 33 g/L H.sub.2 SO.sub.4 and 3 g/L Cu. After a 2 hour leach the final residue containing 59% Cu, 2.3% Ni, 4.5% As and 21% S. Both nickel and arsenic contents were too high for direct feed to a copper smelter.
The first step of the two-stage process disclosed in U.S. Pat. No. 3,616,331 was tested on a similar residue using a leach solution containing 50 to 80 g/L Cu and 3 to 5 g/L. H.sub.2 SO.sub.4 in leach tests at 150.degree. and 200.degree. C. The copper sulphide residue contained 7.9% Ni after a 60 minute leach at 150.degree. C., and 1.6% after a 60 minute leach at 200.degree. C. Even at 200.degree. C. therefore this process did not give the required low level of nickel in the copper sulphide residue, and very little nickel extraction occurred at 150.degree. C.
Neither of the above processes described was satisfactory for treating the specified copper sulphide residue to give a product with sufficiently low nickel and arsenic concentrations for direct smelting to anode copper.